1. Field of the Invention
The present invention relates to removal of heavy metal ions from aqueous wastes, which are produced during electroplating and other industrial processes such as metal finishing, chemical processing, etc. In particular, the invention refers to the removal from wastewaters of the metal ions Cr+3, Ni+2, Cd+2, Cu+2, Zn+2 and other cations capable of forming insoluble ferrites with iron ions.
2. Description of the Related Art
There are a variety of proposed technologies and currently employed industrial processes for the purification of aqueous wastes containing metallic ions A short survey of available processes can be found, for example, in U.S. Pat. No. 5,013,453. Among those methods are dilution, evaporation, alkali-precipitation, absorption, dialysis, electro dialysis, reverse osmosis and ion exchange.
The most commonly used industrial method is alkali-precipitation The method is based on addition of a sufficient amount of base to the waste solution in order to precipitate heavy metals in the form of insoluble metal hydroxides. One disadvantage of this method is associated with the difficulty in choosing a suitable pH for precipitation that is satisfactory for solutions containing variable amounts of different heavy metals. A second problem has to do with the amorphous nature of the hydroxides, which renders their separation from the waste solution difficult, and limits the achievable purification.
The so-called ferrite precipitation process is also described in the literature. This process is based on the addition of iron ions to the wastewater and subsequent adjusting of its pH by the addition of a base. The suspension of hydroxides is then oxidized to form spinel ferrite crystals, which settle from the solution. In this process, heavy metal ions are removed from wastewater because they are incorporated into the crystal structure of the spinel ferrite crystals. The advantages of the ferrite precipitation process are associated with the possibility for simultaneous treatment of various kinds of heavy metal ions, with the environmental stability of the precipitated crystals, with the possibility of relatively simple magnetic separation of the crystals, and with the possibility for reuse of the spinel ferrite as a by-product.
A method of extracting heavy metals from industrial wastewaters by the ferrite precipitation method is disclosed in U.S. Pat. No. 3,931,007. In this method, ferrous ions and a base are added to the wastewater to form a suspension of metal hydroxides. Then an oxidizing gas is bubbled through the suspension to form ferrite crystals, which contain both iron and other heavy metal ions. The precipitated crystals are removed, for example, by magnetic separation to provide wastewater with a very low residual heavy metal concentration. The disadvantage of this method lies in the necessity to bubble the oxidizing gas through the solution. This process requires rather prolonged times (3-10 hours) and therefore renders the whole process costly.
An additional purification method, which employs the formation of ferrites for binding contaminant metals, is described in U.S. Pat. No. 6,238,571. In this method a spent mechanical polishing solution, containing abrasive particles and copper ions is treated by adding ferric and ferrous ions to the solution, then adjusting the pH of the solution by addition of a base such as NH4OH to promote formation of a ferrite precipitate, which can be magnetically separated from the solution. The disadvantage of this method lies in the fact that it requires adjusting the pH to a high value about 11, which is associated with the consumption of a large amount of a base such as potassium hydroxide, ammonium hydroxide or sodium hydroxide.
Furthermore, the described method refers merely to removal of copper and does specify how it should be carried out for the removal of other contaminant ions, for example chrome, which is present in the form of CrO4−2.
In U.S. Pat. No. 5,685,993 an apparatus and method for ferrite formation and removal of heavy metals ions by ferrite co-precipitation from aqueous solutions is described. In accordance with this method, ferromagnetic ferrite crystals can be continuously formed from aqueous waste solution by adding ferrous sulfate to form a solution with a particular molar ratio of iron to heavy metal, agitating the solution, adding a base to the solution to adjust its pH to a value more than 10, distributing air through the solution, forming a ferrite slurry and then settling the slurry. Besides the same disadvantages, which were mentioned in connection with U.S. Pat. No. 3,931,007, this method requires a relatively high value of pH and, relatively long bubbling and retention times which render the process costly. In conclusion it may be stated, that each of the above-listed ferrite precipitation methods has its specific advantages and disadvantages and none of them, unfortunately, provides a satisfactory solution for the treatment of waste solutions containing heavy metals
Thus, despite the fact that numerous processes have been devised for purifying contaminated wastewaters from cations of contaminant heavy metals, there still is a need for a new and improved method, which is simple, inexpensive, convenient, efficient, and reliable. The present invention suggests a new method for the removal of metals from wastewater, which is also based on the formation of ferrites of the metals with their subsequent co-precipitation and separation in the form of an insoluble solid. Because of the unique features of the present invention, the process is simplified and retention times are minimized making the process commercially feasible.
3. Object of the Invention
The object of the present invention is to provide a new and improved method for removal of contaminant heavy metals from wastewater, which overcomes drawbacks of the present methods. In particular the main object of the present invention is to provide a new and improved method, which allows the rapid and efficient treatment of wastewaters containing contaminant heavy metals by co-precipitation of their ferrites. The further object of the present invention is to provide new and better method, which does not require bubbling of a gas oxidizer and which can be carried out in a fast and simple way at ambient temperature. The third object of the present invention is to provide a new and efficient method for the treatment of wastewater, which results in a residual concentration of contaminant metal ions in the wastewater in the range of 0.01-0.04 ppm.
Yet another object of the invention is to provide a new and versatile method for treatment of wastewater, which results in formation of a compact crystalline sediment, which can be easily dried and converted into a dry powder with a high concentration of ferrite suitable for reuse as a ferromagnetic by-product.
The above and other objects and advantages of the present invention can be achieved in accordance with the present method for the removal of contaminant metals ions from aqueous solution by ferrite co-precipitation that occurs in a magnetic field.
In accordance with one of the preferred embodiments the present method comprises:                i) providing a source of iron ions, said source containing at least Fe+2 ions        ii) providing a source of OH− ions        iii) adding said Fe+2 ions to the aqueous solution        iv) adding said OH− ions to the said aqueous solution in the presence of a magnetic field.        v) precipitation of said ferrites from the aqueous solution.        
The present invention has only been summarized briefly.
For a better understanding of additional embodiments of the invention as well of its advantages, reference will now be made to the following description with reference to the accompanying drawings.